This invention relates to a pneumatically inflatable flexible envelope type dam secured to a river bed, and more particularly, to a type thereof having means for preventing the flexible envelope from floating in the water stream during its deflated state.
One of the examples of the conventional inflatable envelope type dam is disclosed in Japanese patent publication Nos. 11702/65 and 2371/69 as shown in FIGS. 1(a) and 1(b), wherein a flexible envelope 1 made of rubberized fabric is inflated with fluid 3 such as air causing the envelope to stand up in a river bed 2 to thus provide a dam. Reference numeral 4 designates the upstream side of the dam.
The dam of this type may have a drawback when water stays at the downstream side of the dam during the deflating state thereof. That is, as shown in FIG. 1(b), residual fluid 3 may remain in the envelope 1 after discharge of the fluid from the envelope 1, so that the dam can not be completely fallen down along the river bed 2 due to the buoyancy given by the residual fluid 3. To be more specific, the residual fluid 3 does exist, since the envelope 1 cannot be completely folded to provide perfect contact between opposing inner surfaces of the envelope due to rigidity of the rubberized fabric. The rigidity provides cylindrical space at the portion to be folded along the transverse direction of the river. The cylindrical space provides an air pillow whose buoyancy is generally larger than the weight of the envelope 1.
Due to this phenomena, it is difficult to convey earth and sand accumulated at the upstream side of the dam far behind the downstream side of the dam when the dam is deflated. Further, earth and sand accumulated at the immediate downstream side of the dam by the deflation cannot be conveyed far behind the dam. Furthermore, there will be a strong probability that the floatingly deflated envelope is damaged or broken by trees etc. which flow down along the river stream. In addition to the above, the deflated envelope may be visually unappealing.
Another example of the conventional inflatable envelope type dam is shown in FIGS. 2(a) and 2(b), wherein buffer members are attached to at least a portion of the inner surface of the envelope and a portion of the dam base in order to protect the envelope from trees or rolling stones etc during the deflating state thereof. In these drawings, a plurality of pockets 5 are attached to a portion of the inner surface of the envelope 1 along the transverse direction of the river in parallel. These pockets 5 are formed of plastic sheet material, cloth or rubberized fabric. A buffer member 6 is disposed in each pocket 5. The buffer members 6 are made of any material capable of providing a shock absorbing effect such as foam material having flexibility. Specifically, PE-foaming material provides excellent shock absorbing effect because of its lightness, high flexibility and durability.
The shock absorbing mats 6 are spread at the position between an imaginary folded line "a" of the envelope 1 and anchors 7 fixed to a foundation member 2 for securing the envelope 1. These mats 6 have a predetermined width and are equally spaced along the inner circumference of the envelope in order to follow up the curvature of the envelope. The width of the shock absorbing mat is determined in light of the flexibility and followability thereof with respect to the movement of the envelope, while the distance between the neighboring mats is determined so that the mats do not interfere with the other during inflation of each envelope.
Further, the shock absorbing mats are disposed in series with a predetermined transverse space "d" along the transverse direction of the river. The length of the mat is determined in light of productivity and installability. The length and space are also determined so as to prevent the dam from generating V-notch (buckling) phenomena during the deflating process in case of the employment of air as the inflating media, which may damage the shock absorbing mats 6.
A dam base 10 is provided with a flexible membrane 11 independent of the envelope 1. The flexible membrane 11 is provided with a pocket 5' into which a shock absorbing mat 6' is disposed. It goes without saying that the manner for providing the buffer members or the positional arrangement thereof is not limited to that disclosed the above.
With this structure, when the flow rate of water is relatively high, the dam can be fallen down substantially completely in its deflating state. However, when the flow rate is relatively low, the portion of the envelope to be folded may float up due to the buoyancy of the buffer members and the residual air. That is, the weight of the dam does not overcome these buoyancies.